The use of large quantities of common sense has long been thought to be critical to the automated understanding of the world. To this end, various groups have collected repositories of common sense in machinereadable form. However, efforts to apply these large bodies of knowledge to enable correspondingly largescale sensor-based understanding of the world have been few. Challenges have included semantic gaps between facts in the repositories and phenomena detected by sensors, fragility of reasoning in the face of noise, incompleteness of repositories, and slowness of reasoning with these large repositories. We show how to address these problems with a combination of novel sensors, probabilistic representation, web-scale information retrieval and approximate reasoning. In particular, we show how to use the 50,000-fact hand-entered OpenMind Indoor Common Sense database to interpret sensor traces of day-to-day activities with 88% accuracy (which is easy) and 32/53% precision/recall (whic...